Process for the preparation of permanent magnetic structures



United States Patent 3,149,408 PROCESS FQR THE TREPAPATIGN 0F PERMA- NENT MAGi *ETTC STRUCTURES Richard B. Fall: and George D. Hooper, Greenviile, Mich assignors to General Electric Company, a corporation of New York No Drawing. Filed Aug. 9, 1962, Ser. No. 215,795 2 Ciaims. (Cl. 229-4556) This invention relates to a process for the preparation of permanent magnetic structures comprising single domain particles of magnetic material.

In the preparation of permanent magnetic structures from fine particles prepared by electrodeposition into a mercury cathode, it is necessary to align or orient the particles to obtain maximum energy products. Alignment of the particles is usually accomplished during a mercury removal step by pressing the slurry of electrodeposited particles and mercury, to which a matrix material has been added, in the presence of a direct current field. The remaining mercury is then removed from the resulting preforms b for example, vacuum distillation and the preforms are then ground into a powder. Each powder fragment contains, after grinding, a large number of aligned, single domain magnetic particles. Since the energy product of the magnet is proportional to its degree of particle orientation, the highly am'sotropic powder must then be realigned when compacted into the final magnetic structure to obtain optimum magnetic properties. The final alignment and compacting of the powder may be carried out in the presence of an A.-C. and D.-C. field by a process more fully set forth in US Patent 2,999,271 which issued on September 12, 1961 and is assigned to the assignee of the present invention.

Alignment of the single domain magnetic particles in a preform, which must be subsequently ground and then recompacted into the final magnet shape, has been con sidered necessary since the individual particles cannot be aligned by means of a magnetic field in a solid matrix. If an isotropic or non-oriented magnet is desired, the preforming and grinding steps may be omitted and the particles compacted in the absence of an electrical field. In the case of anisotropic magnets, it would, however, be desirable in certain instances to eliminate the preforming step without sacrifice of particle orientation in the final magnetic structure.

It is a primary object of the present invention to provide a process for the preparation of permanent magnetic structures of fine particle, single domain magnetic material which does not require a preliminary orientation or" magnetic particles by formation of a preform.

it is an additional object of this invention to provide a simplified process for preparing permanent magnetic structures of fine particle magnetic material which results in improved magnetic properties in the final magnetic structure. Other objects will become apparent from the description which follows.

It has now been found that fine particle permanent magnets may be prepared without a preforming operation by a process which involves the addition of an excess of the matrix material to the particle-mercury slurry immediately preceding the step of mercury removal. The process makes it possible to achieve anisotropic properties in permanent magnet structures without the interposition of a preforming and subsequent grinding operation. Moreover, the properties of the permanent magnets are superior to the equivalent properties obtained from fine particles which have been subjected to a grinding and realigning operation.

In general, the process of this invention comprises electrodepositing single domain magnetic particles into a mercury cathode to form a slurry of electrodeposited parti- Patented Sept. 22, 1964 ICC cles and mercury. A volume of matrix material not substantially less than the volume of mercury in the slurry is then added to the particle-mercury slurry. The mercury is then removed as, for example, by vacuum distillation and the particle-matrix material is pressed in the presence of a D.-C. field at a temperature above the melting point of the matrix into a permanent magnet structure.

The fine particle permanent magnet materials may be electrolytically deposited in a manner more fully set out in US. Patent 2,974,104, dated March 7, 1961, and assigned to the assignee of the present invention. As there disclosed, the process involves the electrolytic deposition of ferromagnetic particles into a mercury cathode from an acidic electrolyte comprising soluble ions of the ferromagnetic material, while maintaining a quiescent interface between the cathode and the electrolyte. The electrolyte or plating solution consists of the soluble salts of the ferromagnetic metals in the form of, for example, their sulfates or chlorides. The pH of the electrolyte is made acidic with, for example, sulphuric or hydrochloric acid, a preferred pH being about 2. The anode may be a consumable anode of the ferromagnetic metal or alloys of several ferromagnetic metals. Alternatively, the anode may be a non-consumable anode of an inert material such as platinum or graphite. A current density between 0.001 amps/cm. and 0.1 amps/cm. may be used although values around 0.02 amps/cm. are preferred. Ordinarily, the electrolyte will be at room temperature of 20 to 30 C. during the electrodeposition although other electrolyte temperatures may be used as suitable adjustments are made in current density and time of deposition. After electrodeposition is completed, the electrodeposited particles are removed as a mercury slurry. The particles may then be heat-treated, to optimize their physical shape and coercive force, for from 1 to minutes at temperatures up to 300 C. and preferably at about 150 to 200 C., and then cooled.

To the slurry of heat-treated, electrodeposited particles and mercury is then added the matrix material in a volume approximately equivalent to the volume of mercury present in the slurry. The matrix material should be added in amounts not substantially less than the volume of mercury. (1n the other extreme, a volume considerably in excess of the volume of mercury, although not ordinarily necessary, may be used. Mercury is removed from this mixture as, for example, by vacuum distillation at a temperature of from about 300 to 400 C. after which the particle-matrix mixture is cooled to room temperature.

The particle-matrix mixture is then placed in a die, heated to a temperature above the melting point of the matrix and a D.-C. field applied to align the particles. The mixture is then pressed to remove excess matrix material and compact the particles into a magnetic structure.

The invention will be more clearly understood by reference to the following example:

EXAMPLE 1 Elongated, single domain magnetic particles of iron and cobalt containing 70% iron30% cobalt were electrolytically deposited into a mercury cathode in accordance with the procedure set forth in the above-referred-to US. Patent 2,974,104. The particle-mercury slurry contained 28.950 lbs. of mercury and 1.050 lb. of elongated, single domain iron-cobalt particles. The slurry was heattreated at a temperature of 200 C. for 10 minutes. 0.30 lb. of a matrix material consisting of a lead-antimony alloy containing 50% lead and 50% antimony was added to the slurry while the slurry was maintained at a temperature of 200 C. for an additional 10 minutes. This mixture was then cooled to room temperature and 30 lbs. of a molten (MP. C.) lead-bismuth (56% bismuth) Element: Pounds Bismuth 16.8

Lead 13.350 Iron 0.734 Cobalt 0. 1 3 6 Antimony 0.150 Mercury 0.150

This material Was placed in a 1 /2" square die and heated to about 175 C. While in a molten state a field of about 7000 gauss was applied through one of the 1 /2" directions. Pressure was applied perpendicular to both 1 /2" dimensions and enough lead-bismuth flowed out between the die and punch clearance to allow the remainder of the material to form the sample identified in Table A below as 11. After obtaining the property data set forth in Table A, the sample Was placed in the same die and the compaction operation was repeated. In this manner samples 12, 1-3, 14, 15, and 16 were made by removing incremental amounts of lead-bismuth from each compact. The table below summarizes the results obtained from these experiments. In Table A, RF. denotes packing fraction, defined as the volume percent of ferromagnetic material present in the specimen. Br is the residual induction and Bis is the instrinsic saturation induction in gauss. The Br/Bis ratio represents the ali i ment of the particles. He is the coercive force in oersteds.

(BH) max is the total magnetic energy in gauss-oersteds.

Table A B1 Br/Bis Ho 1 (BH) max 2,100 75 1,140 .s5x10 2, 300 767 1,175 85X106 3, 200 79 1, 220 1. ssxio 4, 200 844 1, 210 2. 05 1o 5, 200 87a 1, 150 s. 05 10 s, 900 .86 770 3. x10

The above table shows that permanent magnets having acceptable commercial properties may be produced by the process of the invention over a Wide range of packing fractions. In particular, it should be noted that acceptable properties are achieved at packing fractions of less than 0.20. Materials with packing fractions lower than 0.20 are dificult to obtain using conventional preforming and grinding processes because grinding for realignment with low packing fraction materials (relatively large volumes of matrix) ordinarily results in adversely affecting the directionality (Br/Bis) of the permanent magnets.

The invention is particularly useful for the preparation of permanent magnets of iron or iron-cobalt alloys. However, it is also useful With other ferromagnetic materials, including iron, cobalt, nickel, and alloys of iron, cobalt and nickel with each other or with other ferromagnetic alloying constituents or with minor amounts of non-ferromagnetic constituents such as manganese or platinum.

Although lead or lead alloys are a preferred class of matrix materials, the invention may be practiced with a Wide variety of other non-magnetic, metal matrix materials. Other such matrix materials Which are useful, for example, arecadmium, bismuth, silver, or alloys of such metals.

What we claim as new and desire to secure by Letters Patent of the United States is:

l. A process for the preparation of permanent magnetic structures comprising electrodepositing fine magnetic particles into a mercury cathode to form a slurry of electrodeposited particles and mercury,

adding to said slurry a volume of matrix material not substantially less than the volume of mercury in said slurry,

removing the mercury and pressing the particles and matrix at a temperature above the melting point of the matrix in the presence of a 11-0 field to remove excess matrix and compact into a magnetic structure.

2. A process for the preparation of permanent magnetic structures comprising eiectrodepositing fine magnetic particles selected from the group consisting of iron, cobalt, nickel, and alloys of iron, cobalt and nickel, into a mercury cathode to form a slurry of electrodeposited particles and mercury,

heat-treating the electodeposited particles,

adding to said slurry a matrix comprising lead having a volume approximately equal to the volume of mercury remaining in said slurry during said matrix addition, removing the mercury by distillation at reduced pressures and I pressing the particles and matrix at a temperature above the melting point of the matrix material in the presence of a D.-C. field to remove excess matrix and compact into a magnetic structure.

References Cited in the file of this patent UNETED STATES PATENTS 2,758,023 Bareis "Aug. 7, 1956 2,974,104 Paine et al. Mar. 7, 1961 2,990,276 Pirlot June 27, 1961 

1. A PROCESS FOR THE PREPARATION OF PERMANENT MAGNETIC STRUCTURES COMPRISING ELECTRODEPOSITING FINE MAGNETIC PARTICLES INTO A MERCURY CATHODE TO FORM A SLURRY OF ELECTRODEPOSITED PARTICLES AND MERCURY, ADDING TO SAID SLURRY A VOLUME OF MATRIX MATERIAL NOT SUBSTANTIALLY LESS THAN THE VOLUME OF MERCURY IN SAID SLURRY, REMOVING THE MERCURY AND PRESSING THE PARTICLES AND MATRIX AT A TEMPERATURE ABOVE THE MELTING POINT OF THE MATRIX IN THE PRESENCE OF A D.C. FIELD TO REMOVE EXCESS MATRIX AND COMPACT INTO A MAGNETIC STRUCTURE. 